Help to create 2 templates or sensors - I don't know how to start

Configuration
32

That’s due to epsilon error. If you search the term on this forum you’ll see it’s a fairly common issue that can occur with floating-point calculations:
https://community.home-assistant.io/search?q=epsilon%20error

As you’ve discovered, you can use round to control the number of decimal places. Here’s today’s calculated value without rounding:
Screenshot from 2020-11-01 07-56-21

Here’s a version of the Template Sensor that rounds the calculated values to two decimal places:

    my_azimuth_sensor:
      friendly_name: 'My Azimuth Sensor'
      value_template: >-
        {% set x = states('sensor.date') %}
        {% set today = now() %}
        {% set dec21 = now().replace(day=21).replace(month=12).replace(hour=0).replace(minute=0).replace(second=0).replace(microsecond=0) %}
        {% set jun21 = now().replace(day=21).replace(month=6).replace(hour=0).replace(minute=0).replace(second=0).replace(microsecond=0) %}
        {% set previous_dec21 = now().replace(year=now().year-1).replace(month=12).replace(day=21).replace(hour=0).replace(minute=0).replace(second=0).replace(microsecond=0) %}
        {% set jun20_value = 127 + ((jun21 - timedelta(days=1)) - previous_dec21).days * 0.31 %}
        {% if today >= dec21 %}
          {% set y = 127 + (today - dec21).days * 0.31 %}
        {% elif today < jun21 %}
          {% set y = 127 + (today - previous_dec21).days * 0.31 %}
        {% else %}
          {% set y = jun20_value - ((today - jun21).days * 0.31) %}
        {% endif %}    
        {{ y | round(2) }}

Here’s the calculated value it produces:
Screenshot from 2020-11-01 08-04-16

It’s not just one closed parenthesis, it has a matching open parenthesis:

 ((jun21 - timedelta(days=1)) - previous_dec21).days
 ^                                            ^
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